The present invention relates to a failure-free power source device for automatically selecting an AC output matching the degree of stability of an AC power source. More particularly, the present invention is concerned with a failure-free power source device which directly outputs, when an AC power source is stable, an AC input as an AC output or outputs, when an error occurs in the power source, energy stored in an accumulator as the AC output via an inverter.
A failure-free power source device supplies AC power stably to a major load which has to operate continuously even in the event of unexpected power failures. Conventional power source devices of this kind are generally classified into two types, i.e., a constant inverter type device and a constant commercial source type device. The constant inverter type device constantly supplies an AC input from an AC power source to a load as an AC output via a rectifier, accumulator and inverter. The constant commercial source type device has a switching device which supplies, when the AC input is stable, the AC input to the load as the AC output or supplies, on detecting an error in the AC input, the AC input to the load via a rectifier, accumulator and inverter. The constant commercial source type device is disclosed in, for example, Japanese Patent Laid-Open Publication No. 61-247277.
A report entitled "Joint Electricity Study", the Expert Committee for Measures against Momentary Voltage Drops belonging to the Japanese Society for Joint Electricity Study, Vol. 46, No. 3, page 8, FIG. 1-2-1 (issued Feb. 7, 1990) shows the estimated results of yearly mean momentary voltage drop for a single 6.6 kV high-tension distribution line in Japan. This document reports that a momentary voltage drop occurs once a month on the average. Therefore, for loads of the kind not needing stable AC power, the constant commercial source type device will suffice. However, voltage drops in an end power source system are attributable not only to voltage drops on the 6.6 kV distribution line but also to the other loads on a low-tension distribution line and belonging to the same system, as well as wiring distance, problems and troubles derived from power source errors, etc. The constant commercial source type device should, therefore, sometimes be replaced with the constant inverter type device. It follows that to install a conventional failure-free power source device, it is necessary to examine the degree of stability of a commercial power source available at a desired location beforehand and then determine the type of the device matching the location.
Table 1 shown below compares the characteristics of the two types of failure-free power source devices (output capacity of 1.5 kV).
TABLE I ______________________________________ Type Constant Constant Commercial Item Inverter Source ______________________________________ Fluctuation of Input .+-.12% .+-.10% Power Consumption 500W 100W (Including Conversion Loss) Heat 430kcal/h 86kcal/h Noise (Characteristic A) 40dB null Output Voltage & Frequency Accuracy Normal AC volt: .+-.3% dependent on AC Input freq: .+-.0.5% input fluctuation AC Input volt: .+-.3% volt .+-.3% Failure freq: .+-.0.3% freq: .+-.0.5% ______________________________________
As Table 1 indicates, the input voltage fluctuates over a narrower range with the constant commercial source type device than with the constant inverter type device. The constant commercial source type device consumes less power than the constant inverter type device. As for the generation of heat and noise, the constant commercial source type device is advantageous over the constant inverter type device. Further, regarding the accuracy of output voltage and that of output frequency, the constant inverter type device and the constant commercial source type device are comparable in the event of a power failure. However, when the AC input is normal, the constant commercial source type device depends on the stability of the AC input while the constant inverter type device remains the same as at the time of a power failure.
The conventional failure-free power source devices have the following problems left unsolved in addition to the previously mentioned advance examination problem. When the constant commercial source type device is introduced in a power supply system wherein the commercial power source is unstable, the AC output of the device itself becomes unstable. This renders a load faulty, reduces the life of an accumulator due to the frequent discharge of the accumulator, and interrupts the AC output in the event of a power failure due to the short charging of the accumulator. On the other hand, when the constant inverter type device is applied to a power source system wherein the commercial power source is stable, it brings about wasteful power consumption due to conversion losses within the device and the generation of heat and noise.